Disc drives are typical data storage systems and are commonly used in work stations, personal computers, portable computers and other computing systems to store large amounts of data in a form that can be readily available to a user. A disc drive includes a rigid housing having a base and a cover that encloses a variety of components. The components include one or more discs having data surfaces that are coated with a magnetizable medium for storage of digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor that causes the discs to spin and the data surfaces of the discs to pass under respective aerodynamic bearing disc head sliders. The sliders carry magnetic transducers, which write information to and read information from the data surfaces of the discs.
With the rapid increase in areal density of the magnetizable medium, continued, demand exists for disc drives having smaller form factors. A standard disc drive can be as small as a one-inch drive or a sub one-inch drive, such as a 0.85 inch drive. These small form factor disc drive are formed in accordance with CompactFlash® Type II specifications, which have a thickness of 5 mm, and CompactFlash® Type I specifications, which have a thickness of 3.3 mm. One of the difficulties in meeting such small thicknesses is the formation of the base of the small form factor drive.
The base of the disc drive is a complex three-dimensional structure that typically provides a rigid and precise platform upon which multiple mechanical and electronic components are mounted. In general, disc drive bases are made of an aluminum die cast. However, the manufacture of Type I or Type II small form factor disc drives using aluminum die-casting is not ideal for performance and cost reasons. Example problems for an aluminum die cast small form factor base include low material stiffness and a lack of magnetic properties. Typically, small form factor disc drives include a base made at least partially from stamping technologies. Stamping can form larger features of the base, such as motor and base interfaces and pivot and base interfaces. Other more detailed features of the base, such as mounting slots, a filter holder and actuator crash stops are difficult to make by the stamping process alone. In these designs, an overmold plastic injection process is used to create a composite metal/plastic base. In this process, plastic is injected through entrance holes in the base and the plastic is allowed to set while a mold remains in place over the base. The final base includes a metallic stamped section that is fully enclosed by a plastic section such that the plastic grips the metallic stamped portion firmly.
Commonly, the thickness of a stamped base on a small form factor disc drive is between 0.3 and 0.4 mm. As the thickness of small form factor disc drives become smaller, so must the thickness of the base. Attempting to make a small form factor base thinner results in more encounters with related problems. For example, a common problem is base distortion.
Base distortion is distortion or deformation of the metallic section of the base that can likely cause performance problems or functional problems. The metallic section of the base deforms because it has a different heat coefficient than the overmolded plastic section of the small form factor disc drive. The overmold plastic material is injected into the base mold at a high temperature (200 to 300° C.) and cooled quickly to room temperature. Once the base is cooled, both the metallic stamped section and the plastic section will contract. However, the contraction of the plastic section is much larger than the metallic stamped section because the heat coefficient of the plastic section is much higher than that of the metallic stamped section. The difference in contraction causes the plastic section to compress the metallic stamped section and therefore introduce residual stress between the two sections. The residual stress causes the metallic stamped section to distort or deform. The introduction of residual stress is magnified in situations where the base is exposed to low environmental temperatures. Low environmental temperatures cause the plastic section of the base to contract even more.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.